Condensed Consolidated Statements of Cash Flows - USD ($) $ in Thousands	6 Months Ended
	Jun. 30, 2019	Jun. 30, 2018
Operating activities
Net loss	$ (71,770)	$ (30,871)
Adjustments to reconcile net loss to cash used in operating activities:
Non-cash research and development license expense		4,448
Stock-based compensation expense	5,916	2,474
Depreciation and amortization	452	122
Non-cash rent expense	253	(40)
Changes in operating assets and liabilities:
Prepaid expenses and other current assets	(4,048)	(833)
Accounts payable, accrued expenses and other current liabilities	12,273	569
Net cash used in operating activities	(56,924)	(24,131)
Investing activities
Purchases of short-term investments	(69,875)	(20,409)
Maturities of short-term investments	142,687	57,463
Purchases of property and equipment	(3,126)	(10)
Net cash provided by investing activities	69,686	37,044
Financing activities
Net proceeds from issuance of common stock		163,034
Proceeds from the exercise of stock options	519	326
Proceeds from issuance of common stock from ESPP	295
Net cash provided by financing activities	814	163,360
Net increase in cash, cash equivalents and restricted cash	13,576	176,273
Cash, cash equivalents and restricted cash at beginning of period	49,943	34,461
Cash, cash equivalents and restricted cash at end of period	$ 63,519	$ 210,734